Reproductive physiology, embryonic development and sex differentiation are highly sensitive to the impact of environmental stressors, contaminants and diseases. In contemporary research it is sometimes impossible to gain access to appropriate controls in the absence of these gene-expression-altering events. Biological collections hold vast reservoirs of specimens collected since the early 1900’s, that may hold the key to characterising the full extent of the impact of contemporary stressors on vertebrate reproduction. Impact-free historical base-lines will anchor our modern understanding and empower us to estimate the magnitude, mode, tempo and directionality of change. New technology to characterise century-old chromatin architecture and historical RNA (including RNA virus detection) can combine to transform formalin-fixed biological collections into an accurate, comprehensive, and global inventory of gene expression and phenotype. I will discuss how a temporal understanding of gene expression trends advances our understanding of vertebrate reproduction and sex determination, and how this new technology has wide applications for animal conservation, human health, and environmental monitoring.